The present invention relates to satellite radio frequency (RF) beam pointing. In particular, the present invention relates to integrating mechanical and electronic beam pointing in a feedback controlled beam pointing method and apparatus.
Satellites use RF beam pointing techniques to point an antenna at terrestrial and space based targets. The targets may be of interest for space/ground communication, space/space intersatellite links, and for radar beam directed imaging, as examples. Two beam pointing techniques are commonly used: mechanical beam pointing and electronic beam pointing.
Mechanical beam pointing involves mechanically moving or slewing a satellite, or individual antennas on the satellite, to direct the beam generated by the antenna to a particular target. Mechanical pointing can be cost effective for certain applications, but often body and antenna dynamics can result in low to moderate slew rates.
Moreover, because satellites are not perfect rigid bodies, the satellite may take a significant amount of time to dynamically settle, during which beam pointing is relatively inaccurate. Therefore, during the time it takes the satellite and its components to settle, the system is generally non-operational or suffers significant performance degradation. As a general rule for radar satellite imaging systems, imaging is suspended until the pointing error due to dynamic settling of the satellite reaches 1/10 or 1/20 of a beamwidth or less.
Referring now to FIG. 1, the target access regions 102, 104 for a typical synthetic aperture radar ("SAR") imaging satellite are shown. A SAR system relies on relative motion to increase its effective imaging aperture and therefore has difficulties imaging directly below, directly in front, or directly behind the direction of flight. Attenuation and power constraints limit imaging at long distances, near the Earth limb. The result is a "butterfly" instantaneous imaging field-of-regard ("FOR"). In FIG. 1, the FOR is assumed constrained by a 70 degree ground elevation angle (GEA) 106 and a 20 degree GEA 108.
The satellite direction of travel 110 and apparent target motion 112 are also shown.
The target must remain inside the FOR for the duration of the image. Orbits with relatively low altitudes are often desired to reduce radar power, but these orbits also result in rapid (approximately 7 km/sec) relative satellite motion with respect to the ground targets, such that targets remain inside the FOR for relatively short durations (for example, less that one minute). Because multiple targets are often of interest inside the FOR, there is a strong motivation to image each target as quickly as possible.
As will be explained in more detail below with regard to FIGS. 2 and 3, however, mechanical slew induced settling errors prevent the satellite from accurately imaging the target for significant amounts of time. The resolution of each target, the total number of targets that may be imaged in a FOR, and the overall effectiveness of the radar imaging system are correspondingly reduced.
FIG. 2 shows a position error profile 200 for a computer simulation of a mechanical RF beam pointing system used on board a low earth orbit ("LEO") satellite. The position error profile 200 results from the mechanical slew angle profile 300 shown in FIG. 3. The simulation represented in FIG. 3 assumes a RF beamwidth of approximately 0.2 degrees and a 12 second simulated mechanical satellite body slew (beginning at t=0) of 90 degrees to adjust the attitude of the satellite and its rigidly mounted antenna. The settling time required before the pointing accuracy required for nominal operation (approximately 0.01 degree-0.02 degree as indicated by reference numeral 202) was reached was approximately sixteen seconds (from t=12 to approximately t=28).
Thus a significant fraction of the overall available satellite time must be spent waiting for the satellite to slew and settle before capturing images. Unfortunately, precise mechanical pointing with rapid settling is extremely expensive and extremely difficult to implement.
The long slew times and long settling times associated with mechanical pointing systems are not present in electronic pointing systems. Moreover, electronic pointing systems are often more accurate than mechanical pointing systems because jitter and body dynamics associated with mechanical pointing and control hardware are not experienced. However, eliminating all mechanical pointing through the implementation of a broad angle two dimensional (e.g, steerable in azimuth and elevation) phased array is extremely costly and complex.
Primarily, broad angle two dimensional electronic beam pointing is prohibitively expensive because it requires a great number of variable time delay transmit/receive ("TR") modules and RF radiating and receive elements closely spaced together. Furthermore, physical constraints on TR module separation may also limit angular coverage. Another significant drawback of a broad angle two dimensional electronic beam pointing system is the increased backend signal generation and signal processing complexity (as well as increased system power and weight) required to properly operate the two dimension phase array.
Radar is only one example of an application adversely effected by settling errors. As another example, communications applications also suffer from mechanical slew induced settling errors. Because reliable communication requires accurate alignment of transmit and receive antennas, antenna mispointing resulting from settling errors may compromise, as examples, the length of time two entities may communicate, the reliability of the communication, or the rate of communication.
A need has long existed in the industry for a method and apparatus for RF beam pointing with the low cost, broad area coverage features of mechanical pointing and the high accuracy, rapid pointing capability of electronic beam steering.